The present invention relates to a control device for a front and rear wheel drive vehicle and, more particularly, to a control device for a front and rear wheel drive vehicle, in which one of front and rear wheel pairs is driven with an engine and the other one of the front and rear wheel pairs is driven with an electric motor, and on which a torque converter with a lock-up mechanism is mounted.
In recent years, extensive research and development works have been undertaken to provide a front and rear wheel drive vehicle wherein one of front and rear wheel pairs is driven with an engine and the other one of the front and rear wheel pairs is driven with an electric motor. The front and rear wheel drive vehicle is a vehicle which serves as a hybrid vehicle having a low fuel consumption and which serves as a four-wheel drive vehicle having a running through performance.
In general, the front and rear wheel drive vehicle includes a battery that stores electric power to be supplied to the electric motor, and an electric power generator that charges the battery. In a case where the electric motor, which drives the wheels, serves as the electric power generator, the electric motor regenerates a portion of the running energy of the vehicle as an electrical energy, i.e., a regenerative power which is charged into the battery. Usually, the electric motor functions to produce regenerative power during a decelerating condition of the vehicle wherein an accelerator pedal is not depressed. However, in an event that a state of charge of the battery is below a given level, a forced charging operation is carried out even when the accelerator pedal is depressed. During regenerative operation of the electric motor, also, a brake force is applied to the wheels associated with the electric motor on account of its regenerative operation.
In the vehicle equipped with an automatic transmission the fuel efficiency is deteriorated. This is because the torque converter rotates turbine blades by creating oil flow with pump blades rotated by engine drive power, causing a loss of engine output due to the hydraulic friction. Some converters have a lock-up mechanism, which gives a direct connection between the pump side and the turbine side so that the number of revolutions is approximately the same for the both sides. The lock-up mechanism automatically changes the oil flow in the torque converter when the vehicle speed reaches a predetermined speed or more and presses the lock-up clutch mounted on turbine blades to the front cover of pump blades, thereby rotating the turbine blades and pump blades uniformly.
However, when the engine torque is great such as the case where the engine revolution is in a low revolution region, if the slip ratio of the torque converter is set as 100% by controlling the engagement amount of the lock-up clutch, the vibration of an engine will be transferred to a vehicle. The slip ratio of the torque converter is defined as 100% when the lock-up clutch is in a complete direct connection. The transfer of vibration is reduced by slipping the lock-up clutch in such a manner that the slip ratio is reduced from 100% according to the driving state of the vehicle. In the case of the vehicle driven only by the engine, the slip ratio of the torque converter is mapped with the vehicle speed, the throttle opening degree and the like, with which the slip ratio is controlled by adjusting the engagement amount of the lock-up clutch.
However, when said vehicle is equipped with the automatic transmission which has the torque converter with the lock-up mechanism, the closer to 100% the slip ratio of the torque converter is set by controlling the engagement amount of the lock-up clutch, the more will arise the problems associated with NV (Noise Vibration) caused by surging and booming noise during the motor assist mode. Therefore, the transmission loss (namely the loss of engine output) of this vehicle will increase, deteriorating the fuel economy according as the slip ratio of the torque converter is decreased from 100%.
It is therefore an object of the present invention to provide a control device for a front and rear wheel drive vehicle for setting the optimum slip ratio of the torque converter to improve the fuel consumption, avoiding the problems associated with NV during the motor assist mode.
According to the present invention to address the above issues, there is provided a control device for a front and rear wheel drive vehicle, in which one of front and rear wheel pairs is driven with an engine and the other one of front and rear wheel pairs is driven with an electrical motor, and a torque converter with a lock-up mechanism capable of controlling the engagement amount is disposed between said engine and said one of wheel pairs, the control device comprising:
a lock-up control means for controlling said lock-up mechanism so that the target slip amount is set in accordance with driving conditions of said front and rear wheel drive vehicle;
a motor drive power setting means which sets the drive power of said motor;
a compensation means which compensates said target slip amount according to the motor drive power set by said motor drive power setting means.
With this control device for the front and rear wheel drive vehicle, the compensation means compensates the target slip amount during the assistance with the motor drive power. The engagement amount of the lock-up mechanism is increased with the compensation to decrease the slip amount of the converter. When the vehicle is in a starting operation etc., the torque is amplified for the engine drive power by the torque converter, whereas no amplification of the torque is required if there is an assistance with the motor drive power. Therefore, it will not create problems associated with NV even if the slip amount of the torque converter is decreased, as the vibration and noise of the engine decrease during the motor assistance.
According to said control device for the front and rear wheel drive vehicle, said compensation means has the characteristics that said slip amount is controlled so that the number of revolutions and the drive power of said engine do not fall in a certain driving region of said engine.
This control device for the front and rear drive vehicle solves the NV problem by the compensation means, which provides the setting of the optimum slip amount within the range where the relation between the number of engine revolutions and engine drive power do not fall in a given driving region of the engine (surging region, booming noise region and the like).
The driving conditions indicate conditions in regard with running of the front and rear wheel drive vehicle, such as vehicle speed, throttle opening degree and engine torque. And the engine drive power indicates the power associated with the engine in regard with the throttle opening degree or the engine torque etc. And the given driving region of the engine means a problematic driving region related to NV such as surging region and booming noise region etc., which is defined with the number of engine revolutions and the engine drive power depending on the engine characteristics and the like. In addition, the meaning of restricting the compensation of target slip amount is that a maximum number is set for the slip amount to be compensated when the target slip ratio is compensated.